REST API using POST instead of GET

Question

Let's assume a service offers some funcionality that I can use like this:

GET /service/function?param1=value1&param2=value2

Is it right to say that I can use it with a POST query?

POST /service/function { param1 : value1, param2 : value2 }

Are these two queries the same? Can I use the second variant in any case or the documentation should explicitly say that I can use both GET and POST queries?

Answer 1

Just to review, REST has certain properties that a developer should follow in order to make it RESTful:

What is REST?

According to wikipedia:

The REST architectural style describes the following six constraints applied to the architecture, while leaving the implementation of the individual components free to design:

• Client–server: Servers are not concerned with the user interface or user state, so that servers can be simpler and more scalable.
• Stateless: The client–server communication is further constrained by no client context being stored on the server between requests.
• Cacheable: Responses must, implicitly or explicitly, define themselves as cacheable, or not, to prevent clients reusing stale or inappropriate data in response to further requests.
• Layered system: A client cannot ordinarily tell whether it is connected directly to the end server, or to an intermediary along the way. Intermediary servers may improve system scalability by enabling load-balancing and by providing shared caches.
• Code on demand (optional): Servers can temporarily extend or customize the functionality of a client by the transfer of executable code.
• Uniform interface: The uniform interface between clients and servers, discussed below, simplifies and decouples the architecture, which enables each part to evolve independently. (i.e. HTTP GET, POST, PUT, PATCH, DELETE)

What the verbs should do

SO user Daniel Vasallo did a good job of laying out the responsibilities of these methods in the question Understanding REST: Verbs, error codes, and authentication:

When dealing with a Collection URI like: http://example.com/resources/

GET: List the members of the collection, complete with their member URIs for further navigation. For example, list all the cars for sale.

PUT: Meaning defined as "replace the entire collection with another collection".

POST: Create a new entry in the collection where the ID is assigned automatically by the collection. The ID created is usually included as part of the data returned by this operation.

DELETE: Meaning defined as "delete the entire collection".

So, to answer your question:

Is it right to say that I can use it with a POST query? ...

Are these two queries the same? Can I use the second variant in any case or the documentation should explicitly say that I can use both GET and POST queries?

If you were writing a plain old RPC API call, they could technically interchangeable as long as the processing server side were no different between both calls. However, in order for the call to be RESTful, calling the endpoint via the GET method should have a distinct functionality (which is to get resource(s)) from the POST method (which is to create new resources).

Side note: there is some debate out there about whether or not POST should also be allowed to be used to update resources... though i'm not commenting on that, I'm just telling you some people have an issue with that point.